Abstract
We address the mechanisms underlying low-frequency zonal flow generation in a turbulent system through the parametric decay of collisionless trapped particle modes and its feedback on the stabilization of the system. This model is in connection with the observation of barrier transport in reduced gyrokinetic simulations (A. Ghizzo et al., Euro. Phys. Lett. 119(1), 15003 (2017)). Here the analysis is extended with a detailed description of the resonant mechanism. A key role is also played by an initial polarisation source that allows the emergence of strong initial shear flow. The parametric decay leads to the growth of a zonal flow which differs from the standard zero frequency zonal flow usually triggered by the Reynolds stress in fluid drift-wave turbulence. The resulting zonal flow can oscillate at low frequency close to the ion precession frequency, making it sensitive to strong amplification by resonant kinetic processes. The system becomes then intermittent. These new findings, obtained from numerical experiments based on reduced semi-Lagrangian gyrokinetic simulations, shed light on the underlying physics coming from resonant wave-particle interactions for the formation of transport barriers. Numerical simulations are based on a Hamiltonian reduction technique, including magnetic curvature and interchange turbulence, where both fastest scales (cyclotron and bounce motions) are gyro-averaged.
Highlights
An important role of zonal flows (ZFs) in regulating turbulence and transport in tokamaks is broadly accepted
By using a Hamilton-Jacobi formalism using action-angle variables, we focus on the physical mechanism which leads to the formation and the amplification of self-organised transport barriers (TBs) induced by the resonance with oscillating low-frequency zonal flows (LFZFs)
The LFZF can play a key role in the dynamical interaction among ZFs, streamers and shear flows generated by the interchange turbulence
Summary
An important role of zonal flows (ZFs) in regulating turbulence and transport in tokamaks is broadly accepted. TPMs have been obtained by gyro-averaging the particle dynamics over fast scales i.e., over the cyclotron frequency ωcs and bounce ωbs motions in the toroidal geometry, where the index s refers to the considered species s = e, i for electrons or ions respectively. This task is made easier in the framework of the Hamiltonian- Jacobi formalism using action- angle variables (see the works of [26,30] or more generally of [31,32]). Such approach is kinetic in nature and can be reduced to the Hasegawa- Wakatani in a two-field model (see [35] for more details)
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
